Progressive depletion of CD4 T cells is a halmark of untreated acquired immune deficiency syndrome (AIDS), yet the mechanism by which CD4 T cells die during HIV infection remains poorly understood. HIV can directly infect and kill CD4 T cels however too few productively infected cells are present in vivo to explain the masive CD4 T-cell losses that occur. Using primary human lymphoid aggregate cultures (HLAC) formed with tonsil or spleen tissue, we have explored how CD4 T cells die during HIV infection in lymphoid tissues. Our studies have revealed several surprises including: (1) Quiescent bystander CD4 T cells, which are not permissive to productive infection by X4-tropic HIV-1, die in huge numbers as a result of abortive viral infection accompanied by the the accumulation of incomplete viral DNA transcripts in the cytoplasm; (2) Death is not cause by a toxic effect of these DNA or other viral products; rather the CD4 T cells die as a result of a powerful innate immune response launched against the viral DNA, resulting in the production of interferon- and activation of caspase-3 and caspase-1; (3) Ultimately, this response leads to inflammasome assembly and caspase-1 activation triggering the release of bioactive IL-1 and induction of pyroptosis, an intensely inflammatory form of programmed cell death. These events can establish a vicious cycle, whereby dying CD4 T cells release inflammatory mediators that attract additional cells for new rounds of abortive infection and death; 4) Surprisingly, HIV-mediated CD4 T cell depletion by pyroptosis can be completely blocked by specific class of FDA-approved oral sulfonylurea drugs currently used to treat type II diabetes. We now propose a set of pivotal new studies. In Specific Aim 1, we will test whether this mechanism of CD4 T cell depletion also extends to CCR5-expressing CD4 T cells that often display a higher state of cellular activation and thus might be less susceptible to abortive infection. In Specific Aim 2, we propose to identify the cytoplasmic sensor(s) that detects the viral DNA. Multiple sensors may activate the interferon response and the activation of caspase-1 within inflammasomes. In Specific Aim 3, we propose to identify the inflammasome that recruits caspase-1 to process and release IL-1 and to induce pyroptosis. Together, these studies will yield new insights into the mechanism(s) underlying HIV- induced depletion of CD4 T cells-the fundamental problem in AIDS. This new understanding could lead to an entirely new therapeutic approach to AIDS involving interdiction of the host innate immune response that promotes both CD4 T cell death and inflammation. This approach could form a strong adjunct to traditional antiviral therapy and might be particularly beneficial for patients exhibiting broad drug resistance or rapid progression of disease. PUBLIC HEALTH RELEVANCE: These studies seek to understand a fundamental problem in HIV/AIDS, namely how the virus causes the progressive depletion of CD4 T cells. Success in these studies will not only provide new insights into the molecular underpinnings of HIV immunopathogenesis, but also could provide exciting new approaches for preserving CD4 T cells and inhibiting inflammation thereby blocking HIV-induced disease progression. These approaches could be particularly valuable for patients who have failed multiple antiviral drug regimens or who display rapidly progressive disease.